Hydrogen isotope discrimination in aquatic primary producers: implications for aquatic food web studies

Large differences in δ ²H of primary producers between aquatic and terrestrial ecosystems are used to identify subsidies, discriminate organic matter sources, and reduce uncertainty in food web studies. Previous investigations of hydrogen isotope ratios suggest there may be predictable differences between the δ ²H of water and organic matter for different types of primary producers. We define the difference in the net isotopic discrimination between water and bulk organic matter (om) as: Δ_H = (δ ²H_om ? δ ²H_water) ÷ (1 + δ ²H_water ÷ 1,000). We summarized Δ_H values from published literature and we measured the δ ²H of water and primary producers in order to compare Δ_H among aquatic and terrestrial primary producers. Measurements were made from three water body types (lake, river, coastal lagoon) and their associated watersheds. Although we predicted a large and equivalent net isotopic discrimination for aquatic primary producers, we found considerable variability among groups of aquatic producers. Macroalgae, benthic microalgae, and phytoplankton had more negative Δ_H values (i.e. greater isotopic discrimination) than both aquatic macrophytes and terrestrial vegetation. The more positive δ ²H_om and hence lower Δ_H of terrestrial vegetation was expected due to relative increases in the heavier isotope, deuterium, during transpiration. However, the more positive values of δ ²H_om and relatively low Δ_H in aquatic macrophytes, even submerged species, was unexpected. Marine macroalgae had high variability in δ ²H_om as a group, but low variability within distinct species. Variability among types of primary producers in δ ²H_om and in Δ_H should be assessed when hydrogen is used in isotopic studies of food webs.     

A multi-isotopic and trace element investigation of the Cretaceous-Tertiary boundary layer at Stevns Klint, Denmark - inferences for the origin and nature of siderophile and lithophile element geochemical anomalies

Os, Sr, Nd and Pb isotope data were collected from a profile across the Cretaceous-Tertiary (K-T) boundary layer at Stevns Klint, Denmark. ?_Nd [T=65 Ma] values from within the boundary layer (Fish Clay) are lower by ∼1 ? unit than those of the underlying Maastrichtian limestone and the overlying Danian chalk sequences. Systematic profile-upward changes of Pb, Sr and Os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the Fish Clay. While Os, Nd and Pb isotopes indicate the admixing of less radiogenic components to the Fish Clay, Sr isotopes show elevated radiogenic values in the boundary layer, relative to the carbonate sequences beneath and above it. The sudden change in lithophile (e.g., Sr, Pb and Nd) isotope compositions at the base of the Fish Clay and profile-upward trends of ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios towards those of the overlying Danian chalk are interpreted to reflect recovery from enhanced, acid rain-induced continental (local?) weathering input to the seawater. However, a continental crustal source is invalid for the siderophile element Os. In the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements (PGEs) and certain moderately siderophile elements (Cr, Ni, Co, V) in K-T boundary sediments, including Stevns Klint [Shukolyukov and Lugmair, Science 282 (1998) 927-929], and supported by the finding of projectile debris [Bauluz et al., Earth Planet. Sci. Lett. 182 (2000) 127-136] and the occurrence of abundant Ni-rich spinel at many K-T sites [Robin et al., Nature 363 (1993) 615-617; Kyte, Nature 396 (1998) 237-239], we favor to explain the sudden drop of ¹⁸⁷Os/¹⁸⁸Os ratios from 0.210 to 0.160 at the K-T boundary to derive from global fall-out of extraterrestrial matter. The present ¹⁸⁶Os/¹⁸⁸Os ratio of 0.119836±0.000004 measured in the basal layer of the Fish Clay is within the uncertainty a chondritic value. We therefore exclude the possibility of a major contribution of PGEs to the sediment from iron meteorites. Chondrite-normalized (Ru/Ir)_N ratios of ∼0.95±0.14 and (Os/Ir)_N ratios of ∼0.93±0.14 in the Fish Clay cannot distinguish between abundance ratios of different types of chondrites, and strongly sub-chondritic (Pt/Ir)_N ratios of ∼0.62±0.09 (2σ) suggest differential PGE remobilization through the sedimentary column (and consequently the alteration of inter-element ratios). PGEs and the moderately siderophile elements Cr, Ni, V, and Co form an elemental association with systematically upward-decreasing concentrations in the Fish Clay. Low Co/Ni ratios of ∼0.12 in the Fish Clay relative to values of ∼0.35 in the over- and underlying carbonate sequences support mixing of meteorite-derived (Co/Ni ∼0.05) and terrestrial upper mantle/crustal (Co/Ni >∼0.3) sources. While lithophile element isotope data indicate an increased continental crustal input to the Fish Clay at the K-T transition, the uncertainty with respect to possible post-depositional alteration of abundance patterns of siderophile and moderately siderophile elements - though not affecting the chondritic isotopic composition of Os - does not allow confirmation of indications from chromium isotopes for a carbonaceous (CV-type) meteorite as the preferred K-T impactor type by Shukolyukov and Lugmair [Science 282 (1998) 927-929].     

Carbon isotopic compositions of organic matter across continental Cretaceous–Tertiary (K–T) boundary sections: Implications for paleoenvironment after the K–T impact event

To assess the environmental perturbation induced by the impact event that marks the Cretaceous–Tertiary (K–T) boundary, concentrations and isotopic compositions of bulk organic carbon were determined in sedimentary rocks that span the terrestrial K–T boundary at Dogie Creek, Montana, and Brownie Butte, Wyoming in the Western Interior of the United States. The boundary clays at both sites are not bounded by coals. Although coals consist mainly of organic matter derived from plant tissue, siliceous sedimentary rocks, such as shale and clay, may contain organic matter derived from microbiota as well as plants. Coals record δ¹³C values of plant-derived organic matter, reflecting the δ¹³C value of atmospheric CO₂, whereas siliceous sedimentary rocks record the δ¹³C values of organic matter derived from plants and microbiota. The microbiota δ¹³C value reflects not only the δ¹³C value of atmospheric CO₂, but also biological productivity. Therefore, the siliceous rocks from these sites yields information that differs from that obtained previously from coal beds.Across the freshwater K–T boundary at Brownie Butte, the δ¹³C values decrease by 2.6‰ (from − 26.15‰ below the boundary clay to − 28.78‰ above the boundary clay), similar to the trend in carbonate at marine K–T sites. This means that the organic δ¹³C values reflect the variation of δ¹³C of atmospheric CO₂, which is in equilibrium with carbon isotopes at the ocean surface. Although a decrease in δ¹³C values is observed across the K–T boundary at Dogie Creek (from − 25.32‰ below the boundary clay to − 26.11‰ above the boundary clay), the degree of δ¹³C-decrease at Dogie Creek is smaller than that at Brownie Butte and that for marine carbonate.About 2‰ decrease in δ¹³C of atmospheric CO₂ was expected from the δ¹³C variation of marine carbonate at the K–T boundary. This δ¹³C-decrease of atmospheric CO₂ should affect the δ¹³C values of organic matter derived from plant tissue. As such a decrease in δ¹³C value was not observed at Dogie Creek, a process that compensates the δ¹³C-decrease of atmospheric CO₂ should be involved. For example, the enhanced contribution of ¹³C-enriched organic matter derived from algae in a high-productivity environment could be responsible. The δ¹³C values of algal organic matter become higher than, and thus distinguishable from, those of plant organic matter in situations with high productivity, where dissolved HCO₃⁻ becomes an important carbon source, as well as dissolved CO₂. As the δ¹³C-decrease of atmospheric CO₂ reflected a reduction of marine productivity, the compensation of the δ¹³C decrease by the enhanced activity of the terrestrial microbiota means that the microbiota at freshwater environment recovered more rapidly than those in the marine environment.A distinct positive δ¹³C excursion of 2‰ in the K–T boundary clays is superimposed on the overall decreasing trend at Dogie Creek; this coincides with an increase in the content of organic carbon. We conclude that the K–T boundary clays include ¹³C-enriched organic matter derived from highly productive algae. Such a high biological productivity was induced by phenomena resulting from the K–T impact, such as nitrogen fertilization and/or eutrophication induced by enhanced sulfide formation. The high productivity recorded in the K–T boundary clays means that the freshwater environments (in contrast to marine environments) recovered rapidly enough to almost immediately (within 10 yr) respond to the impact-related environmental perturbations.     

Extraction of soluble collagen and its feasibility in the palaeodietary research

In current palaeodietary research, gelatinization is the main method to extract insoluble collagen(ISC) from ancient bones. However, the degradation products of ISC, i.e., soluble collagen(SC), is often neglected and abandoned. In this work, we try to separate the extracts of ancient bones using gel chromatography and compare the contents of carbon and nitrogen, atomic C/N ratio, and stable carbon and nitrogen isotopic values of the extracts from three peaks to determine which peak can be attributed to SC. At last, the potential application of SC in palaeodietary research is discussed based on the comparison of stable isotopic values between ISC and SC. Among the three peaks, the second with the retention time between 17.5 min and 27.5 min had the most broad peak shape, indicating that the molecular weights of proteins collected were most variable. Besides, the contents of carbon and nitrogen and atomic C/N ratio of extracts in this peak were closest to the corresponding ISC. Based on the above, we conclude that the extract in second peak is SC. More important, the δ 13C and δ 15N values of ISC and SC are very similar. For ISC and SC with atomic C/N ratios within the normal range(2.9–3.6), the mean difference of δ 13C value was only(0.3±0.2)‰(n=2) while δ 15N value was(0.6±0.1)‰(n=2). Although the atomic C/N ratios of some SC are slightly beyond the normal range, the mean differences of δ 13C and δ 15N values were still only(0.4±0.1)‰ and(0.3±0)‰(n=2) respectively. These isotopic differences are quite below the isotope fractionation in one trophic level(δ 13C values of 1‰–1.5‰ and δ 15N values of 3‰–5‰), suggesting that SC had great application potentials in palaeodietary research.     

Molecular and isotopic characterization of the particulate organic matter from an eutrophic coastal bay in SE Brazil

The present work aimed at studying the origin of particulate organic matter in Guanabara Bay and in some rivers of the Guanabara basin by using elemental composition, isotopic ratios (δ¹³C and δ¹⁵N) and molecular markers (sterols) in samples collected in two periods (winter and summer). Elemental and isotopic compositions were determined by dry combustion and mass spectrometry, respectively, while sterols were investigated by GC–FID and GC–MS. Higher sterol concentrations were present in the north-western part of the bay in winter (5.10–23.5 μg L^–1). The high abundance of algal sterols (26–57% of total sterols), the elemental composition (C/N=6–8) and the isotopic signatures (δ¹³C=−21.3‰ to −15.1‰ and δ¹⁵N=+7.3‰ to +11.1‰) suggested the predominance of autochthonous organic matter, as expected for an eutrophic bay, although seasonal variation in phytoplankton activity was observed. Coprostanol concentration (fecal sterol) was at least one order of magnitude higher in the particulate material from fluvial samples (4.65–55.98 μg L^–1) than in the bay waters (<0.33 μg L^–1). This could be ascribed to a combination of factors including efficient particle removal to sediments in the estuarine transition zone, dilution with bay water and bacterial degradation during particle transport in the water column.     

Comparison of C and N stable isotope ratios between surface particulate organic matter and microphytoplankton in the Gulf of Lions (NW Mediterranean)

Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) in surface water and 63–200 μm-sized microphytoplankton collected at the fluorescence maximum were studied in four sites in the Gulf of Lions (NW Mediterranean), a marine area influenced by the Rhone River inputs, in May and November 2004. Some environmental (temperature, salinity) and biological (POM, Chlorophyll a and phaeopigments contents, phytoplankton biomass and composition) parameters were also analysed. Significantly different C and N isotopic signatures between surface water POM and microphytoplankton were recorded in all sites and seasons. Surface water POM presented systematically lower δ¹³C (∼4.2‰) and higher δ¹⁵N (∼2.8‰) values than those of microphytoplankton, due to a higher content of continental and detrital material. Seasonal variations were observed for all environmental and biological parameters, except salinity. Water temperature was lower in May than in November, the fluorescence maximum was located deeper and the Chlorophyll a content and the phytoplankton biomass were higher, along with low PON/Chl a ratio, corresponding to spring bloom conditions. At all sites and seasons, diatoms dominated the phytoplankton community in abundance, whereas dinoflagellate importance increased in autumn particularly in coastal sites. C and N isotopic signatures of phytoplankton did not vary with season. However, the δ¹⁵N of surface water POM was significantly higher in November than in May in all sites likely in relation to an increase in ¹⁵N/¹⁴N ratio of the Rhone River POM which influenced surface water in the Gulf of Lions. As it is important to determine true baseline values of primary producers for analysing marine food webs, this study demonstrated that C and N isotopic values of surface water POM cannot be used as phytoplankton proxy in coastal areas submitted to high river inputs.     

Isotopic evidence for meteoric-hydrothermal alteration of plutonic igneous rocks in the Yakutat Bay and Skagway areas,Alaska

Oxygen and hydrogen isotope ratios were measured on coexisting minerals from quartz diorites and quartz monzonites from a section across the Coast Range batholith in the Skagway area, Alaska, including a variety of outlying plutons west of the batholith in the Yakutat Bay-Mt. St. Elias region (latitudes 59–60°N). The extremely low and variable δ¹⁸Oand δD results indicate widespread meteoric-hydrothermal alternaiton of the Coast Range batholith, and to a lesser extent, of the Yakutat Bay plutons as well. In the Yakutat Bay area, the plutons with K—Ar ages younger than 50 m.y. have widely varying δD values of ?72 to ?148, compared to δD = ?69to?90 for all but one sample in the 50–225 m.y. age grouping (one biotite has δD = ?109). This suggests that the major meteoric-hydrothermal episodes in this area occurred during the Eocene and Miocene. This involved relatively small meteoric water/rock ratios(<0.1), as none of the δ¹⁸O values show any clear-cut evidence of alteration (δ¹⁸O_quartz= 7.4 ?11.8; δ¹⁸O_feldspar= 5.7?10.0). However, in the section across the Coast Range batholith, 85% of the plutonic rocks have very low δD values of ?100 to ?167, and the δ¹⁸O values are extremely variable δ¹⁸O_feldspar= + 10.3to?4.0 and Δ¹⁸O_{quartz-feldspar}= 0.4?10.5. These data indicate that a major portion of the batholith, particularly the quartz monzonite-rich eastern part, but also including many of the quartz diorite plutons as well, interacted with meteoric-hydrothermal convective systems that involved water/rock ratios of about 0.3–1.4. The quart diorite plutons are most depleted in¹⁸O near their northeast contacts against the younger quartz monzonite intrusions. The primary igneous δ¹⁸O values of the quartz diorites were apparently higher than those of the quartz monzonites; they are also unusually high in¹⁸O compared to most other analyzed quartz diorites, suggesting derivation from, exchange with, or assimilation of high-¹⁸O metasediments or altered volcanic rocks. These data and conclusions are very similar to those reached previously on a similar isotopic study of the Coast Range batholith in British Columbia, 700 km to the southeast at latitudes 54–55°N, except that in the Skagway area an even greater proportion of the batholith was apparently depleted in deuterium. This implies that deep (?5km?) circulation of meteoric groundwaters is probably a characteristic of the later stages of emplacement of the Cordilleran batholiths of western North America, suggesting that the eastern sections of these batholiths in particular were emplaced at relatively shallow depths.     

Stable carbon and hydrogen isotopes of gases from the large tight gas fields in China

By the end of the year 2010,a total of 15 large tight gas fields have been found in China,located in the Ordos,Sichuan,and Tarim basins.The annual production and total reserves of these fields in 2010 were 222.5×108and 28657×108m3,respectively,accounting for 23.5%and 37.3%,respectively,of the total annual production and reserves of natural gases in China.They took a major part of all natural gas production and reserves in China.According to the analyses of 81 gas samples,the stable carbon and hydrogen isotopic compositions of tight gases in China have following characteristics:(1)Plots of 13C1-13C2-13C3,13C1-C1/C2+3and 13C1-13C2demonstrate the coal-derived origin of tight gases in China;(2)For the primary alkane gases,both carbon and hydrogen isotopic values increase with increasing molecular mass,i.e.,13C113C213C313C4and2H12H22H3;(3)The isotopic differences of 13C2-13C1,13C3-13C1,2H2-2H1and 2H3-2H1decrease with increasing Ro(%)and C1/C1–4;(4)There are seven causes for the carbon and hydrogen isotopic reversal,however,the carbon and hydrogen isotopic reversal of tight gases in China is caused mainly by multiple stages of gas charge and accumulation.     

Distribution and sources of sedimentary organic matter in a tropical estuary,south west coast of India (Cochin estuary): A baseline study

Surface sediments samples were collected from 9 stations of the Cochin estuary during the monsoon, post-monsoon and pre-monsoon seasons and were analyzed for grain size, total organic carbon (OC), total nitrogen (TN) and stable isotopic ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) to identify major sources of organic matter in surface sediments. Sediment grain size is found to be the key factor influencing the organic matter accumulation in surface sediments. The δ¹³C values ranges from ?27.5‰ to ?21.7‰ in surface sediments with a gradual increase from inner part of the estuary to the seaward side that suggest an increasing contribution of marine autogenous organic matter towards the seaward side. The δ¹⁵N value varies between 3.1‰ and 6.7‰ and it exhibits complex spatial and seasonal distributions in the study area. It is found that the dynamic cycling of nitrogen through various biogeochemical and organic matter degradation processes modifies the OC/TN ratios and δ¹⁵N to a considerable degree. The fraction of terrestrial organic matter in the total organic matter pool ranges from 13% to 74% in the surface sediments as estimated by δ¹³C based two end member mixing model.     

Investigating the distribution and sources of organic matter in surface sediment of Coombabah Lake (Australia) using elemental,isotopic and fatty acid biomarkers

Extensive physical and biological measurements were made of the surface sediments within the shallow, semi-urbanised Coombabah Lake in southern Moreton Bay, Australia. Sediment bulk parameters (C/N ratios, δ¹³C and δ¹⁵N) and fatty acid biomarkers were used to determine distributions and sources of organic matter in the intertidal sediments. The determination of organic matter sources within coastal and estuarine settings is important in understanding the roles of organic matter as energy and nutrient sources. Spatial variability of biomarker values within the sediments were interpreted by thematic maps employing the Krigging algorithm. Grain size analysis indicated the lake was dominated by mud (<63 μm) in the southern (landward) and sand (>63 μm) in the northern (seaward) lake regions, respectively. Surface sediment organic C and N values ranged from 0.12% to 1.76% and 0.01% to 0.12% dry weight, respectively, and C/N ratios averaged 16.3±3.19%. Sedimentary δ¹³C values ranged from −26.1‰ to −20.9‰, with an average value of −23.9±1.0‰. Sedimentary δ¹⁵N values ranged from +1.7‰ to +4.8‰, with an average value of +2.8±0.8‰. Bulk sediment parameters suggested that sedimentary organic matter is provided predominantly by allochthonous sources in the form of fringing mangroves. Thirty-nine individual fatty acids were identified using gas chromatography–mass spectrometry. The mean contributions of long chain fatty acids (LCFAs), polyunsaturated fatty acids (PUFAs), saturated fatty acids (SAFAs) and bacterial fatty acids (BAFAs) were, respectively, 13.9±11.4%, 7.6±4.1%, 53.6±8.6% and 18.2±4.6% of the identified fatty acid methyl esters (FAMEs), with BAFAs occurring in all sampled sediments. Fatty acid compositions varied throughout lake sediments, which indicated spatial differences in autochthonous and allochthonous organic matter sources, including terrestrial and planktonic (i.e. zooplankton, diatoms and other algal species) sources. The contribution of organic matter from shoreline mangroves was confirmed by the presence of LCFAs and 18:2ω6 and 18:3ω3, which are markers for mangroves in this ecosystem. BAFAs were identified in increased proportions in sediments adjacent to urban developments and dominated by mud. Grain size was identified as a dominant factor in the fatty acid compositions and contributing values to FAME pool. Spatial patterns of C/N ratios, δ¹³C and δ¹⁵N values, and fatty acid biomarker contributions illustrated that there is a greater contribution of autochthonous and labile organic matter to the sedimentary organic matter pool in the northern (marine entrance) sediments compared to the more allochthonous sourced organic matter of the southern region of the lake. This study details the distribution and sources of organic matter within Coombabah Lake and illustrates the usefulness of a multiple biomarker approach in discriminating organic matter sources within estuarine environments.     

Nitrogen abundances and isotopic compositions in stony meteorites

Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ¹⁵N of+30to+50%.; (2) enstatite chondrites have δ¹⁵N of?30to?40‰; (3) C3 chondrites have low δ¹⁵N with large internal variations; (4) ordinary chondrites have δ¹⁵N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ¹⁵N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.     

Isotopic lead ages of chondritic meteorites

The isotopic composition of lead was determined for two carbonaceous, two H, and two L chondrites. All are falls. The²⁰⁶Pb/²⁰⁴Pb ratios cover a range from 9.45 to 37.33; the²⁰⁷Pb/²⁰⁴Pb ratios range from 10.39 to 26.10. The isotopic data define a²⁰⁷Pb/²⁰⁶Pb age of 4.635 AE. Uranium and lead concentration data indicate that the isotopic lead ages for the Bruderheim chondrite are concordant within approximately 20%. This contrasts with lead data in the literature for chondrites, which consistently indicate discordant isotopic lead ages due to large excesses of radiogenic lead by factors of two or more. The isotopic lead ages for Pultusk may be concordant; those for a sample of Richardton are not. The lack of concordance for Richardton is not due to the analytical procedures, rather it is likely a result of the handling history of the chondrite prior to analysis.The L-3 chondrite, Mezo-Madaras contains such a high concentration of lead - 5.27 PPM - that accurate ratios for primordial lead can be obtained. These are ²⁰⁶Pb/²⁰⁴Pb= 9.310;²⁰⁷Pb/²⁰⁴Pb= 10.296, values which are in close agreement with a recently reported measurement on lead in troilite from the Canyon Diablo iron meteorite.     

Spatial distributions of δ13C, δ15N and C/N ratios in suspended particulate organic matter of a bay under serious anthropogenic influences: Daya Bay,China

Stable isotopic signatures (δ¹³C and δ¹⁵N) and C/N ratios of suspended particulate organic matter (POM) were investigated from the surface water of Daya Bay during summer and winter of 2015. The relatively high δ¹³C_POM values suggested the input of ¹³C-depleted terrigenous organic matter was low in Daya Bay. There were significant correlations between δ¹³C_POM values and chlorophyll a concentrations both during summer and winter, suggesting the δ¹³C_POM values were mainly controlled by the phytoplankton biomass in the surface water. The distribution of δ¹⁵N_POM values was more complicated than that of δ¹³C_POM and displayed low values in the outer bay and the Dan'ao River estuary. ¹⁵N-depleted ammonia originating from industrial wastewater might have strongly influenced the water quality and stable isotopic signatures of POM near the Dan'ao River estuary. The δ¹³C_POM and δ¹⁵N_POM values strongly reflect the influences of anthropogenic activity and eutrophication in Daya Bay.     

The oxygen isotope record in Murchison and other carbonaceous chondrites

The ranges of δ¹⁸O and δ¹⁷O in components of the Murchison (C2) chondrite exceed those in all other meteorites analyzed. Previous authors have proposed that C2 chondrites are the products of aqueous alteration of anhydrous silicates. A model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes. The minimum number (two) of initial isotopic reservoirs is assumed. Two major stages of reservoir interaction are required: one at high temperature to produce the¹⁶O-mixing line observed for the anhydrous minerals, and another at low temperature to produce the matrix minerals. The isotopic compositions severely constrain the conditions of the low-temperature process, requiring temperatures < 20°C and volume fractions of water > 44%. Extension of the model to the data on C1 chondrites requires aqueous alteration in a warmer, wetter environment.     

Nitrogen contents and isotopic ratios of clasts from the enstatite chondrite Abee

Nitrogen contents and isotopic ratios have been determined for three clasts from the enstatite chondrite Abee by stepwise heating. The clasts possess a wide range in nitrogen content, ranging from 254 to 850 ppm, whereas the nitrogen isotopic ratios are nearly identical atδ¹⁵N= ?29.2±0.6‰. A refractory inorganic nitrogen-bearing phase contains about 90% of the nitrogen which is released at temperatures of 1000°C and above. The stepwise heating experiments suggest the possible existence of two other distinct nitrogen components, released at low (770°C) and high (1500°C) temperatures.     

Evidence from isotopic geochemistry as an indicator of eutrophication of Meiliang Bay in Lake Taihu,China

In this paper, Lake Taihu, a large shallow freshwater lake in China, is chosen as an example of reconstruction of eutrophication through the comparison between stable isotopes from dissolved nutrients and plants and water column nutrient parameters and integration of multiple proxies in a sediment core from Meiliang Bay including TN, TP, TOC, C/N, δ ¹⁵N, δ ¹³C, etc. Differences in aquatic plant species and trophic status between East Taihu Bay and Meiliang Bay are indicated by their variations in δ ¹³C and δ ¹⁵N of aquatic plants and δ ¹⁵N of NH₄ ⁺. A significant influence of external nutrient inputs on Meiliang Bay is reflected in temporal changes in δ ¹⁵N of NH₄ ⁺ and hydro-environmental parameters. The synchronous change between δ ¹³C and δ ¹⁵N values of sedimented organic matter (OM) has been attributed to elevated primary production at the beginning of eutrophication between 1950 and 1990, then recent inverse correlation between them has been caused by the uptake of ¹⁵N-enriched inorganic nitrogen by phytoplankton grown under eutrophication and subsequent OM decomposition and denitrification in surface sediments, indicating that the lake has suffered from progressive eutrophication since 1990. Based on the use of a combination of stable isotopes and elemental geochemistry, the eutrophication of Meiliang Bay in Lake Taihu could be better traced. These transitions of the lake eutrophication respectively occurring in the 1950s and 1990s have been suggested as a reflection of growing impacts of human activities, which is coincident with the instrumental data.     

Re-assessing the nitrogen signal in continental margin sediments: New insights from the high northern latitudes

Organic and inorganic nitrogen and their isotopic signatures were studied in continental margin sediments off Spitsbergen. We present evidence that land-derived inorganic nitrogen strongly dilutes the particulate organic signal in coastal and fjord settings and accounts for up to 70% of the total nitrogen content. Spatial heterogeneity in inorganic nitrogen along the coast is less likely to be influenced by clay mineral assemblages or various substrates than by the supply of terrestrial organic matter (TOM) within eroded soil material into selected fjords and onto the shelf. The δ¹⁵N signal of the inorganic nitrogen (δ¹⁵N_inorg) in sediments off Spitsbergen seems to be appropriate to trace TOM supply from various climate- and ecosystem zones and elucidates the dominant transport media of terrigenous sediments to the marine realm. Moreover, we postulate that with the study of sedimentary δ¹⁵N_inorg in the Atlantic–Arctic gateway, climatically induced changes in catchment's vegetations in high northern latitudes may be reconstructed. The δ¹⁵N_org signal is primarily controlled by the availability of nitrate in the dominating ocean current systems and the corresponding degree of utilization of the nitrate pool in the euphotic zone. Not only does this new approach allow for a detailed view into the nitrogen cycle for settings with purely primary-produced organic matter supply, it also provides new insights into both the deposition of marine and terrestrial nitrogen and its ecosystem response to (paleo-) climate changes.     

Classification and origin of natural gases from Lishui Sag,the East China Sea Basin

Natural gases discovered up to now in Lishui Sag, the East China Sea Basin, differ greatly in gaseous compositions, of which hydrocarbon gases amount to 2%–94% while non-hydrocarbon gases are dominated by CO₂. Their hydrocarbon gases, without exception, contain less than 90% of methane and over 10% of C₂ ⁺ heavier hydrocarbons, indicating a wet gas. Carbon isotopic analyses on these hydrocarbon gases showed that δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃ are basically lighter than ?44‰, ?29‰ and ?26‰, respectively. The difference in carbon isotopic values between methane and ethane is great, suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation. δ ¹³ \(C_{CO_2 } \) values of nonhydrocarbon gases are all heavier than ?10‰, indicating a typical abiogenic gas. The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit, consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter. Moreover, δ ¹³C₁ values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰ heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit while δ ¹³C₂ and δ ¹³C₃ values of the former are over 9‰ heavier than those of the latter. Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag, where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter, therefore, natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.     

Cenozoic evolution of the sulfur cycle: Insight from oxygen isotopes in marine sulfate

We report new data on oxygen isotopes in marine sulfate (δ¹⁸O_SO4), measured in marine barite (BaSO₄), over the Cenozoic. The δ¹⁸O_SO4 varies by 6‰ over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The δ¹⁸O_SO4 does not co-vary with the δ³⁴S_SO4, emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the δ¹⁸O_SO4 over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The δ¹⁸O_SO4 also does not co-vary with the δ¹⁸O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the δ¹⁸O_SO4 to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the δ¹⁸O_SO4. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.     

Temporal variations of C,N, δC,and δN in organic matter collected by a sediment trap at Cuenca Alfonso,Bahía de La Paz,SW Gulf of California

To monitor the composition and the vertical flux of particulate matter from the sea surface, a sediment trap was moored in Cuenca Alfonso, Bahía de La Paz, a zone of high productivity in the southwestern Gulf of California. Carbonate-free samples from 2002 to 2005 were analyzed for C_org, N, δ¹³C, and δ¹⁵N. The results show seasonal and interannual variability, with the δ¹³C and δ¹⁵N values larger in spring and summer than in fall and winter. The C:N ratio and δ¹³C increased by 1.5 units from 2002 to 2003–2005, suggesting a change in the supply of organic matter and-or the use or preferential degradation of N_org. There was no interannual variation in δ¹⁵N. The occasional high δ¹⁵N values suggest that physical mechanisms, such as the shoaling and advection into the bay of ¹⁵N-rich subsurface equatorial water, occur over short time periods. The latter is presumed to be related to the periodic development of a significant cyclonic gyre in the southern Gulf.